There is a need for all power supplies connected to the mains to meet the harmonic limits of the European standard EN-61000-3-2 or similar in other countries. There are further needs to meet efficiency standards, e.g. 80 PLUS and Energy Star, in future. Prior arts which can meet the EN-61000-3-2 requirements can be divided into two categories.
The first category (U.S. Pat. No. 4,437,146, U.S. Pat. No. 5,134,355, U.S. Pat. No. 5,654,880, U.S. Pat. No. 6,900,623, US2006/0158912) senses the rectified AC voltage and controls the operation of the converter using a feedback loop such that the line input current drawn by the converter follow the rectified AC voltage. This category suffers from the problem of feedback loop stability when the AC line voltage varies over a wide range, e.g. from 115 AV to 240 VAC. They use complicated compensation network to ensure stability and is susceptible to noise and distortion in the rectified AC voltage. For example, the prior art U.S. Pat. No. 6,900,623 senses the RMS value of the AC line voltage and scales the loop gain accordingly. This category uses a multiplier to generate a reference sinusoidal signal. However, it is hard to have a good multiplier which can operate at large input signal and this problem bounds the best achievable power factor.
The second category (U.S. Pat. No. 5,867,379, U.S. Pat. No. 5,742,151) uses a nonlinear carrier signal without sensing the rectified AC voltage to generate the same control signal as in the first category. However, they are applicable to limited type of converters only.
Both categories use Pulse Width Modulation (PWM) to control the switch inside the AC/DC converter and suffer from the problem of concentrated EMI. There are studies in prior arts on the merit of using Pulse Frequency Modulation (PFM) or Frequency Modulation (FM) instead of PWM control to alleviate the EMI problem such that a smaller and cheaper EMI filter can be used instead. However, these prior arts use a dedicated unit to adjust the switching frequency while the pulse width is controlled by another unit. Thus the complexity of the design becomes double.
Lastly, in order to meet efficiency standards, e.g. the 80 PLUS and Energy Star, extra circuitry is needed to override the normal control of these power factor controllers and reduce the power consumption at light loading. This implies extra circuitry and more complex control to ensure the controllers transit smoothly between the light loading mode and normal loading mode.
The PFM approach is a well known solution to provide smooth transition between the light loading mode and normal loading mode. However, conventional AC/DC converters which do not has a Power Factor Correction front stage will suffer from the problem of larger output ripple at medium load. However, this is not a concern for two-stage AC/DC converter which has a Power Factor Corrector as the front stage and a DC/DC converter as the second stage.
Thus, there is a need to combine the functions of power factor correction and the PFM into a single controller to overcome all problems in the prior arts.